1. Field of the Invention
This invention relates to the insulation and containment system of tanks designed for the storage or transportation of liquid cryogens, and more particularly, those of the membrane type in which the cold liquid is contained in a flexible membrane and its hydrostatic pressure is transmitted by means of a rapid insulation to a load bearing vessel structure remaining at a temperature close to ambient.
2. Description of Prior Art
Bulk Storage and transportation of natural gas, ethylene, nitrogen and other industrial gases, in liquid state under atmospheric pressure and at a cryogenic temperature has been done using storage tanks and tanker ships of various designs, and various safety regulations have been imposed for the design, construction, and use of such tanks or vessels within the United States ports and coastal waters. Liquid Natural Gas boils at a temperature of approximately -160.degree. C. To minimize the atmospheric boiling and vaporization of such a cold liquid it is necessary to reduce to a very low level the heat influx from the ambience to the liquid. This is achieved by a thick layer of thermally insulating material. Insulating materials such as perlite, glass wool and most plastic foams do not have any appreciable strength. Accordingly, they can only be used on the outside of a cold tank which contains the liquid and is capable of withstanding all static and dynamic liquid loads. Such self supporting tanks are generally made of thick welded plates of High Nickel Alloy steel or of Aluminum Alloys. The cryogenic metals have the property of remaining relatively ductile at cryogenic temperatures so that they are less subject to the propagation of fractures than other metals which become brittle at the same temperatures. Because of the high material and welding costs of such thick wall self supporting tanks, another kind of cryogenic liquid containment system is preferably used, which is generally called a membrane cryogenic tank.
In such a system, the insulating material is rigid and has sufficient compressive strength to transmit the static and dynamic liquid loads to an outer tank structure which remains at or near ambient temperature. For that reason, the load bearing tank structure can be made of less expensive, ordinary carbon steel. Such savings are partially offset however by the greater safety requirements imposed by the location of the insulation layer inside a warm tank made of non-cryogenic steel. Because the insulation is made of a rigid material it is subjected to large thermal stresses when its inner face is at -160.degree. C. and the outer face at ambient temperature.
Unless they are relieved by flexible foam expansion joints, such thermal stresses, added to the compressive stress due to static and dynamic liquid loads may cause insulation layers to crack. In such an event, the cryogenic liquid would penetrate into the insulation and would cause a drastic lowering of temperature at some points of the load bearing tank structure. Because the material selected for the construction of such outer tank structure is subject to brittle fracture at low temperature, the tank would fail and the cryogenic liquid might spill out, with potentially catastrophic consequences. To reduce such a hazard, all membrane cryogenic tanks are required to include two successive leak proof barriers. The primary barrier, generally made of a flexible thin gauge cryogenic metal covering the insulation layer, is in direct contact with the cryogenic liquid and must not be liquid-tight but also to a large degree gas tight. The secondary barrier, often located within the insulation layer must also be liquid-tight so as to contain the cryogenic liquid in the event of failure of the primary barrier.
Any liquid leaking past the primary barrier receives sufficient heat from the ambience to vaporize. The resulting cold gas must be vented and the presence of the leak immediately detected. For this reason the space between the primary and secondary barriers, generally called interbarrier space is swept by a circulation of cold gas exhausting into a venting system. The interspace between the secondary barrier and the load bearing structure called the insulation space, is also swept by a gas circulation, the purpose of which is not only to provide additional venting of gas in the event of a leak, but also, in the case of cryogenic tanker ships, to detect any entry of water from the adjacent ballast compartments into the cryogenic tank, resulting from any failure of the double hull of the tanker. Gas permeable insulation space in known designs is created by means of a lattice of discontinuous wood grounds bolted against the tank wall using welded studs and to which the primary insulation panels separated by expansion joints are glued. Known construction methods of membrane cryogenic tanks are based on assembling successively, inside a completed tank structure up to five layers made of small elements constituting the respective parts of membrane containment systems:
(1) the insulation space, including monitoring channels PA1 (2) the secondary load bearing insulation PA1 (3) the secondary barrier (liquid tight) PA1 (4) the primary cryogenic insulation and interbarrier spaces PA1 (5) the primary barrier (gas tight membrane).
Each of the individual elements of such parts of known membrane containment systems, (studs, grounds, washers, nuts, insulation panels, expansion joints, barrier joints, primary insulation panels, membrane panels, membrane joints, etc.) is of sufficiently small dimensions and light weight that it can be handled, positioned and fastened by one or two men using portable tools. This method of construction employs a large number of men working simultaneously from a multi-stage scaffolding covering the entire inner surface of the tank, which scaffolding is later removed.
The presence of a large number of workers inside the closed tank requires the installation in the tank of adequate ventilation systems, which are also subsequently removed. The only access for bringing into the closed tank all such elements of the containment system, workers with their portable tools, and all parts of the removable scaffolding and ventilation system is through the hatch openings on the tank tops.
Conventional design of the ship tank structure places stringent limitations on the maximum dimensions of such openings, which control some of the dimensions of said elements and parts, and limit the circulation of men and materials. Throughout the construction of the tank and during the subsequent dismantling and removal of the multi-stage scaffolding and of the ventilation systems great care must be taken to avoid the accidental drop of any element, hand tool, scaffolding or ventilation part onto the lower areas of the cryogenic tank, which would be severely damgaged by the impact of such falling objects.
The known methods of construction of membrane tanks, in successive layers, also require sequential testing of the assembled elements at each step of the construction of the containment system pulling tests on studs, levelling and alignment of grounds arranged in a rectangular grid, compression of foam expansion joints, leak proofing of all joints of the secondary and primary barriers, etc.). The assembly of a large number of small elements, builtup in successive layers and sequentially tested, by a large number of men working on a multi-stage scaffolding, which characterizes such known methods of construction, applicable to known membrane containment systems, results in very high labor costs and in a lengthy construction time, which increases the financial burden associated with the immobilization of capital invested in the unfinished ship, tanks, and construction facilities.
To reduce the high cost of membrane-type cryogenic tanks, various wet wall systems have been described, in which at least one of the liquid-tight barriers is omitted, for instance the primary metallic barrier. In that case, the safety of the system is entirely dependent upon the pressure equilibrium between the liquid hydrostatic load and the vapor pressure of a small amount of liquid trapped in the inner surface of the insulating foam, and partially vaporized. A major drawback of these wet wall systems is the presence of combustible gas in the insulating walls of the tanks when in service, which makes it difficult to gas-free them sufficiently for inspection or repairs. Such wet wall systems do not fall into the category of membrane cryogenic tanks, recognized by ship classification society and by the U.S. Coast Guard. They do not provide the same redundant safety features as the true membrane systems, namely a primary and secondary barrier, the integrity of which can be monitored for the containment of the cold liguid.